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Abstract:

An orthopaedic tibial prosthesis includes a tibial baseplate sized and
shaped to cover substantially all of a resected proximal tibial surface,
and a tibial bearing component sized to leave a posteromedial portion of
the tibial baseplate exposed when the tibial bearing component is mounted
to the baseplate. The exposed posteromedial portion of the tibial
baseplate includes a chamfered profile which cooperates with a
correspondingly chamfered profile at a posteromedial edge of the tibial
bearing component to create a substantially continuous chamfer extending
from the resected tibial surface to the medial articular surface of the
tibial bearing component. Advantageously, this chamfer leaves an absence
of material (i.e., a relief or void) at the posteromedial edge of the
tibial prosthesis, thereby enabling deep flexion of the prosthesis
without impingement between the tibial prosthesis and adjacent anatomic
tissues or prosthetic structures.

Claims:

1. A tibial bearing component comprising: an inferior surface; an
opposing superior surface defining a lateral articular surface and a
medial articular surface; an anteroposterior axis disposed between said
lateral articular surface and said medial articular surface and extending
from an anterior edge to a posterior edge of said tibial bearing
component; and a peripheral wall extending from said inferior surface to
said superior surface, said peripheral wall having a tibial bearing
chamfer extending from a posterior medial edge of said superior surface
toward said inferior surface, said tibial bearing chamfer extending
across at least 25% of an available proximal/distal distance between said
superior and said inferior surfaces at said posterior medial edge, said
tibial bearing chamfer forming an acute bearing chamfer angle with said
inferior surface such that said bearing chamfer extends proximally and
anteriorly from said inferior surface toward said superior surface.

2. The tibial bearing component of claim 1, wherein said tibial bearing
chamfer extends across substantially the entire available
proximal/distance.

3. The tibial bearing component of claim 1, wherein said acute angle
comprises an angle between about 35 degrees and about 75 degrees.

4. The tibial bearing component of claim 1, wherein said acute angle
comprises an angle of about 61 degrees.

5. The tibial bearing component of claim 1, wherein said bearing chamfer
defines an overall anteroposterior extent, measured in a sagittal plane
as an anteroposterior distance along a direction parallel to said
anteroposterior axis, of about 2.7 mm.

6. The tibial bearing component of claim 1, wherein said bearing chamfer
defines an overall length, measured in a sagittal plane as a distance
from an anterior/proximal end of said bearing chamfer to an opposing
posterior/distal end of said bearing chamfer, of at least about 4 mm.

7. The tibial bearing component of claim 1, wherein said bearing chamfer
defines an arcuate profile in a sagittal plane.

8. The tibial bearing component of claim 7, wherein said bearing chamfer
defines a first radius, measured in the sagittal plane, of between about
5 mm and about 180 mm.

9. The tibial bearing component of claim 8, wherein said first radius
defines an anteroposterior extent, measured in the sagittal plane as an
anteroposterior distance along a direction parallel to said
anteroposterior axis, of about 2.0 mm.

10. The tibial bearing component of claim 8, wherein said bearing chamfer
defines a second radius, measured in the sagittal plane, of between about
5 mm and about 180 mm.

11. The tibial bearing component of claim 10, wherein said first radius
defines an anteroposterior extent, measured in the sagittal plane as an
anteroposterior distance along a direction parallel to said
anteroposterior axis, of about 0.7 mm.

12. The tibial bearing component of claim 1, wherein said bearing chamfer
defines a linear profile in a sagittal plane.

13. The tibial bearing component of claim 1, further comprising a lateral
chamfer extending from a lateral posterior edge of said superior surface
toward said inferior surface.

14. The tibial bearing component of claim 1, in combination with a
femoral component adapted to articulate with said tibial bearing
component, a relief defined between said femoral component and said
bearing chamfer when said femoral component is configured in a deep
flexion orientation with respect to said tibial bearing component.

15. The tibial bearing component of claim 1, further comprising a rounded
transition from said medial articular surface to an adjacent medial edge
of said peripheral wall, said rounded transition defining a transition
radius of at least about 0.45 mm.

16. The tibial bearing component of claim 1, further comprising a rounded
transition from said medial articular surface to an adjacent medial edge
of said peripheral wall, said rounded transition defining an arc length
of at least about 0.83 mm.

17. The tibial bearing component of claim 1, further comprising a rounded
transition from said lateral articular surface to an adjacent lateral
edge of said peripheral wall, said rounded transition defining a
transition radius of at least about 0.5 mm

18. The tibial bearing component of claim 1, further comprising a rounded
transition from said lateral articular surface to an adjacent lateral
edge of said peripheral wall, said rounded transition defining an arc
length of at least about 0.9 mm.

19. The tibial bearing component of claim 1, in combination with a tibial
baseplate comprising: a bone contacting surface; an opposing baseplate
superior surface defining a medial compartment and a lateral compartment;
a baseplate anteroposterior axis disposed between said lateral
compartment and said medial compartment, said baseplate anteroposterior
axis extending from an anterior edge to a posterior edge of said tibial
baseplate; and a baseplate peripheral wall extending from said bone
contacting surface to said baseplate superior surface, said baseplate
peripheral wall including a baseplate chamfer extending from a posterior
portion of said medial compartment to said bone contacting surface, said
baseplate chamfer defining an acute baseplate chamfer angle with said
bone contacting surface.

20. The combination of claim 19, wherein said baseplate chamfer angle is
substantially identical to bearing chamfer angle, such that said
baseplate chamfer and said tibial bearing chamfer cooperate to form a
substantially continuous chamfer extending from said bone contacting
surface to said medial articular surface when said tibial bearing
component is mounted to said tibial baseplate.

21. The combination of claim 19, wherein said baseplate chamfer angle
comprises an angle between about 35 degrees and about 75 degrees.

22. A tibial prosthesis kit, the kit comprising: a tibial baseplate
including medial and lateral compartments bounded by a baseplate
periphery, said medial compartment including a posteromedial baseplate
potion defining a baseplate chamfer, said baseplate chamfer defining an
acute baseplate chamfer angle with respect to a coronal plane; a first
tibial bearing component comprising: a first inferior surface sized to
fit within said baseplate periphery; an opposing first superior surface;
a first medial portion having a first medial articular surface forming
part of said first superior surface; a first lateral portion disposed
opposite said first medial portion with respect to a first
anteroposterior axis, said first lateral portion having a first lateral
articular surface forming another part of said first superior surface;
and a first bearing chamfer extending from a posterior medial edge of
said first superior surface toward said first inferior surface, said
first bearing chamfer extending across at least 25% of a first available
proximal/distal distance between said first superior and first inferior
surfaces at said posterior medial edge, said first bearing chamfer
defining an acute first bearing angle with respect to said first inferior
surface; and a second tibial bearing component comprising: a second
inferior surface sized to fit within said baseplate periphery; an
opposing second superior surface defining a second lateral articular
surface and a second medial articular surface; and a second medial
portion having a second medial articular surface forming part of said
second superior surface; a second lateral portion disposed opposite said
second medial portion with respect to a second anteroposterior axis, said
second lateral portion having a second lateral articular surface forming
another part of said second superior surface; and a second bearing
chamfer extending from a posterior medial edge of said second superior
surface toward said second inferior surface, said second bearing chamfer
extending across at least 25% of a second available proximal/distal
distance between said second superior and second inferior surfaces at
said posterior medial edge, said second bearing chamfer defining an acute
second bearing angle with respect to said second inferior surface, said
second tibial bearing component differently sized from said first tibial
bearing component.

23. The kit of claim 22, wherein: said first tibial bearing component
defines a first overall bearing thickness defined between said first
inferior surface and said first superior surface, said differently sized
second tibial bearing component defines a second overall bearing
thickness defined between said second inferior surface and said second
superior surface, and said second overall bearing thickness is greater
than said first overall bearing thickness.

24. The kit of claim 22, wherein said tibial baseplate comprises a first
tibial baseplate, the kit further comprising a second tibial baseplate
differently sized from said first tibial baseplate, wherein: said medial
portion of first bearing component defines a first overall medial
anteroposterior extent, as measured along a direction parallel to said
first anteroposterior axis of said first tibial bearing component; said
medial portion of second bearing component defines a second overall
medial anteroposterior extent, as measured along a direction parallel to
said second anteroposterior axis of said second tibial bearing component;
and said second overall medial anteroposterior extent is greater than
said first overall medial anteroposterior extent.

25. The kit of claim 22, wherein said first bearing angle and said second
bearing angle are each defined by a proximal/anterior end of said first
bearing chamfer and said second bearing chamfer, respectively, said first
bearing angle equal to said second bearing angle.

26. The kit of claim 22, wherein said first bearing chamfer defines a
first radius, measured in a sagittal plane, and said second bearing
chamfer defines a corresponding second radius in the sagittal plane, said
first radius different from said second radius.

27. The kit of claim 26, wherein said first radius is larger than said
second radius.

28. The kit of claim 26, wherein both of said first radius and said
second radius define a common anteroposterior extent as measured in the
sagittal plane.

29. The kit of claim 28, wherein said common anteroposterior extent is
about 2.0 mm.

30. The kit of claim 26, wherein said first bearing chamfer defines a
third radius disposed distally of said first radius, and said second
bearing chamfer defines a corresponding fourth radius disposed distally
of said second radius, said third radius different from said fourth
radius.

31. The kit of claim 30, wherein both of said third radius and said
fourth radius define a common anteroposterior extent as measured in the
sagittal plane.

32. The kit of claim 31, wherein said common anteroposterior extent is
about 0.7 mm.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under Title 35, U.S.C.
§119(e) of U.S. Provisional Patent Application Ser. No. 61/381,800,
filed on Sep. 10, 2010 and entitled TIBIAL PROSTHESIS FACILITATING
ROTATIONAL ALIGNMENT, the entire disclosure of which is hereby expressly
incorporated by reference herein.

[0005] Orthopaedic prostheses are commonly utilized to repair and/or
replace damaged bone and tissue in the human body. For example, a knee
prosthesis used in total knee arthroplasty may include a tibial baseplate
that is affixed to a resected or natural proximal tibia, a femoral
component attached to a resected or natural distal femur, and a tibial
bearing component coupled with the tibial baseplate and disposed between
the tibial baseplate and femoral component. Knee prostheses frequently
seek to provide articulation similar to a natural, anatomical
articulation of a knee joint, including providing a wide range of
flexion.

[0006] The tibial bearing component, sometimes also referred to as a
tibial insert or meniscal component, is used to provide an appropriate
level of constraint and conformity at the interface between the femoral
component and the tibial bearing component. For a knee prosthesis to
provide a sufficient range of flexion with a desirable kinematic motion
profile, the tibial bearing component and tibial baseplate must be sized
and oriented to interact appropriately with the femoral component of the
knee prosthesis throughout the flexion range. Substantial design efforts
have focused on providing a range of prosthesis component sizes and
shapes to accommodate the natural variability in bone sizes and shapes in
patients with orthopaedic prostheses, while preserving flexion range and
desired kinematic motion profile.

[0007] In addition to facilitating implantation and providing enhanced
kinematics through manipulation of the size and/or geometry of prosthesis
components, protection and/or preservation of soft tissues in the natural
knee joint is also desirable.

[0008] A given prosthetic component design (i.e., a tibial baseplate,
tibial bearing component, or femoral component) may be provided to a
surgeon as a kit including a variety of different sizes, so that the
surgeon may choose an appropriate size intraoperatively and/or on the
basis of pre-surgery planning. An individual component may be selected
from the kit based upon the surgeon's assessment of fit and kinematics,
i.e., how closely the component matches the natural contours of a
patient's bone and how smoothly the assembled knee joint prosthesis
functions in conjunction with adjacent soft tissues and other anatomical
structures. Soft tissue considerations include proper ligament tension
and minimization of soft tissue impingement upon prosthetic surfaces, for
example.

[0009] In addition to prosthetic sizing, the orientation of a prosthetic
component on a resected or natural surface of a bone also impacts
surgical outcomes. For example, the rotational orientation of a tibial
baseplate and tibial bearing component with respect to a resected
proximal tibia will affect the interaction between the corresponding
femoral prosthesis and the tibial bearing component. Thus, substantial
design efforts have been focused on providing prosthetic components which
are appropriately sized for a variety of patient bone sizes and are
adapted to be implanted in a particular, proper orientation to achieve
desired prosthesis performance characteristics.

SUMMARY

[0010] The present disclosure provides an orthopaedic tibial prosthesis
including a tibial baseplate sized and shaped to cover substantially all
of a resected proximal tibial surface, and a tibial bearing component
sized to leave a posteromedial portion of the tibial baseplate exposed
when the tibial bearing component is mounted to the baseplate. The
exposed posteromedial portion of the tibial baseplate includes a
chamfered profile which cooperates with a correspondingly chamfered
profile at a posteromedial edge of the tibial bearing component to create
a substantially continuous chamfer extending from the resected tibial
surface to the medial articular surface of the tibial bearing component.
Advantageously, this chamfer leaves an absence of material (i.e., a
relief or void) at the posteromedial edge of the tibial prosthesis,
thereby enabling deep flexion of the prosthesis without impingement
between the tibial prosthesis and adjacent anatomic tissues or prosthetic
structures.

[0011] To facilitate selection of proper prosthesis components, a set of
trial tibial baseplate components are provided, with each component in
the set sized to substantially cover various sizes of a proximal tibial
surface exposed after resection. Each trial component has a perimeter
that is substantially identical to the perimeter of correspondingly sized
tibial baseplate, and is therefore larger than the corresponding tibial
bearing component at the posteromedial portion owing to the void created
by the posteromedial chamfer. The trial components include visual
indicators of this posteromedial void, thereby establishing a visual
acuity between the trial components and the final assembled tibial
prosthesis. This visual acuity promotes surgeon confidence that the trial
components are appropriately paired with their smaller counterpart
permanent tibial bearing components.

[0012] In an alternative embodiment, the permanent tibial baseplate may be
symmetrical or otherwise differently-shaped from the trial component. The
asymmetric trial component may still be used to determine proper
rotation, sizing, and orientation of the permanent component, as above,
but may then be replaced with the differently-shaped tibial baseplate for
final implantation. Where such a differently-shaped tibial baseplate is
used, the trial component may include visual indication of the disparity
between the trial periphery and the baseplate periphery. This visual
indication of disparity promotes surgeon confidence in the final
implanted position and orientation of the baseplate.

[0013] Proper rotational orientation of the baseplate and tibial bearing
components is assessed by comparing one or more of the trial components
to the natural resected tibial surface. To ensure that this rotational
orientation is properly transferred to the permanent components, the
trial components provide drill guide holes which can be used to locate
and orient the proper location for one or more mounting holes for the
permanent tibial baseplate. The corresponding tibial baseplate is then
provided with fixation pegs formed at the same location relative to the
baseplate periphery. Alternatively, the provisional component may include
a central aperture corresponding to a stem or keel formed on the tibial
baseplate.

[0014] In one form thereof, the present invention provides a tibial
bearing component comprising: an inferior surface; an opposing superior
surface defining a lateral articular surface and a medial articular
surface; an anteroposterior axis disposed between the lateral articular
surface and the medial articular surface and extending from an anterior
edge to a posterior edge of the tibial bearing component; and a
peripheral wall extending from the inferior surface to the superior
surface, the peripheral wall having a tibial bearing chamfer extending
from a posterior medial edge of the superior surface toward the inferior
surface, the tibial bearing chamfer extending across at least 25% of an
available proximal/distal distance between the superior and inferior
surfaces at the posterior medial edge, the tibial bearing chamfer forming
an acute bearing chamfer angle with the inferior surface such that the
bearing chamfer extends proximally and anteriorly from the inferior
surface toward the superior surface.

[0015] In another form thereof, the present invention provides a tibial
prosthesis kit, the kit comprising: a tibial baseplate including medial
and lateral compartments bounded by a baseplate periphery, the medial
compartment including a posteromedial baseplate potion defining a
baseplate chamfer, the baseplate chamfer defining an acute baseplate
chamfer angle with respect to a coronal plane; a first tibial bearing
component comprising: a first inferior surface sized to fit within the
baseplate periphery; an opposing first superior surface; a first medial
portion having a first medial articular surface forming part of the first
superior surface; a first lateral portion disposed opposite the first
medial portion with respect to an anteroposterior axis, the first lateral
portion having a first lateral articular surface forming another part of
the first superior surface; and a first bearing chamfer extending from a
posterior medial edge of the first superior surface toward the first
inferior surface, the first bearing chamfer extending across at least 25%
of a first available proximal/distal distance between the first superior
and first inferior surfaces at the posterior medial edge, the first
bearing chamfer defining an acute first bearing angle with respect to the
first inferior surface; and a second tibial bearing component comprising:
a second inferior surface sized to fit within the baseplate periphery; an
opposing second superior surface defining a second lateral articular
surface and a second medial articular surface; and a second medial
portion having a second medial articular surface forming part of the
second superior surface; a second lateral portion disposed opposite the
second medial portion with respect to an anteroposterior axis, the second
lateral portion having a second lateral articular surface forming another
part of the second superior surface; and a second bearing chamfer
extending from a posterior medial edge of the second superior surface
toward the second inferior surface, the second bearing chamfer extending
across at least 25% of a second available proximal/distal distance
between the second superior and second inferior surfaces at the posterior
medial edge, the second bearing chamfer defining an acute second bearing
angle with respect to the second inferior surface, the second bearing
component differently sized from the first bearing component.

[0016] In yet another form thereof, the present invention provides a
method of determining a tibial prosthesis size, the method comprising:
providing a trial component having a void indicator; placing the trial
component on a resected proximal tibial surface to create a buffer zone
on all sides between a perimeter of the tibial surface and a perimeter of
the trial component, the void indicator occupying a posteromedial area of
the tibial surface when the trial component is placed on the tibial
surface; removing the trial component; providing a tibial baseplate
having a posteromedial baseplate chamfer; and implanting the tibial
baseplate on the resected proximal tibia so that the baseplate chamfer
occupies the posteromedial area.

[0017] In one aspect, the method further includes: providing a tibial
bearing component having a posteromedial tibial bearing chamfer; and
mounting the tibial bearing component on the tibial baseplate so that the
tibial bearing chamfer and the baseplate chamfer form a substantially
continuous chamfer.

[0018] In another aspect, the relief created by the chamfer prevents
impingement of a femoral component, femur or soft tissues upon the tibial
base plate chamfer in a deep flexion orientation corresponding to at
least 155 degrees of flexion.

[0019] In still another form thereof, the present invention provides a
family of tibial prostheses, the prostheses comprising: a plurality of
trial components, each of the trial components comprising: a different
size and geometrical arrangement defining a trial component perimeter,
the geometrical arrangement including asymmetry about an anteroposterior
axis; and a posteromedial area having a void indicator; a plurality of
tibial baseplates having a bone-contacting surface and a superior
surface, each of the bone-contacting surfaces defining a baseplate
perimeter that is substantially identical to a respective one of the
trial component perimeters; and a plurality of tibial bearing components,
each of the tibial bearing components having a tibial bearing component
perimeter that is substantially identical to a respective one of the
trial components perimeters excluding the posteromedial area.

[0020] In one aspect, the anteroposterior axis is a home axis, the home
axis defined as a line extending from a posterior point at the geometric
center of an attachment area between a posterior cruciate ligament and
the tibia, to an anterior point disposed on an anterior tubercle of the
tibia, the tubercle having a tubercle width W, the anterior point
disposed on the tubercle at a location medially spaced from a peak of the
tubercle by an amount equal to W/6.

[0021] In another aspect, the void indicator comprises one of a
contrasting color, contrasting texture, contrasting surface finish, and a
geometric discrepancy.

[0022] In still another form thereof, the present invention provides a
tibial prosthesis kit, the kit comprising: a tibial baseplate including a
baseplate posteromedial portion with a baseplate chamfer formed thereon;
a tibial bearing component including a tibial bearing posteromedial
portion with a tibial bearing chamfer formed thereon, the tibial bearing
component adapted to mount to the tibial baseplate to form a tibial
prosthesis, the baseplate chamfer and the tibial bearing chamfer
cooperating to define a gap between a posteromedial periphery the tibial
baseplate and a corresponding posteromedial periphery the tibial bearing
component when the tibial bearing component is attached to the tibial
baseplate; and a plurality of trial components having means for
identifying the gap.

[0023] In one aspect, the means for identifying the gap comprises one of a
contrasting color, contrasting texture, contrasting surface finish, and a
geometric discrepancy.

[0024] In still another form thereof, the present invention provides a
tibial prosthesis kit, the kit comprising: a tibial baseplate defining a
baseplate periphery, said tibial baseplate having a means for fixation to
a bone; a trial component defining an asymmetric periphery different from
said baseplate periphery, said trial component having at least one
locator hole corresponding to the location of the means for fixation,
said trial component having a void indicator indicating the location of
portions of said asymmetric periphery not present in said baseplate
periphery.

[0025] In still another form thereof, the present inventor provides a
method of determining a tibial prosthesis size, the method comprising:
providing a trial component defining a trial component periphery and
having a void indicator within the trial component periphery; placing the
trial component on a resected proximal tibial surface such that the void
indicator occupies an area of the tibial surface when the trial component
is placed on the tibial surface; removing the trial component; providing
a tibial baseplate having a baseplate periphery that is different from
said trial component periphery; and implanting the tibial baseplate on
the resected proximal tibia so that the baseplate periphery occupies an
area on the proximal tibia that corresponds to the trial component
periphery with the void indicator removed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above-mentioned and other features and advantages of this
invention, and the manner of attaining them, will become more apparent
and the invention itself will be better understood by reference to the
following description of embodiments of the invention taken in
conjunction with the accompanying drawings, wherein:

[0027] FIG. 1A is an exploded, perspective view of a tibial baseplate and
tibial bearing component in accordance with the present disclosure;

[0028] FIG. 1B is a perspective view of the tibial baseplate and tibial
bearing component shown in FIG. 1A;

[0029] FIG. 2A is a top plan view of a resected proximal tibial surface,
with a prosthetic tibial baseplate component and tibial bearing component
of FIGS. 1A and 1B mounted thereon;

[0030] FIG. 2B is a schematic view of a periphery of the tibial baseplate
component shown in FIG. 2A;

[0031] FIG. 3A is a sagittal elevation, section view of one embodiment of
the tibial prosthesis shown in FIG. 2A, taken along line 3A-3A;

[0032] FIG. 3B is an enlarged, partial view of the tibial prosthesis shown
in FIG. 3A, illustrating a posteromedial chamfer;

[0033] FIG. 3C is a coronal elevation, section view of another embodiment
of the tibial prosthesis shown in FIG. 2A, taken along line 3C-3C;

[0034] FIG. 3D is an enlarged, partial view of the tibial prosthesis shown
in FIG. 3C, illustrating a medial transition from an articular surface to
a bearing periphery;

[0035] FIG. 4A is a sagittal elevation, section view of another embodiment
of the tibial prosthesis shown in FIG. 2A, taken along line 4A-4A;

[0036] FIG. 4B is an enlarged, partial view of the tibial prosthesis shown
in FIG. 4A, illustrating a posteromedial chamfer;

[0037] FIG. 5 is a top plan view of the resected proximal tibial surface
shown in FIG. 2A, with a properly sized tibial trial component thereon;

[0038] FIG. 6 is a side, elevation view of the tibia and trial component
shown in FIG. 2; and

[0039] FIG. 7 is a side, elevation view of the tibia and prosthetic
components shown in FIG. 4.

[0040] Corresponding reference characters indicate corresponding parts
throughout the several views. The exemplifications set out herein
illustrate exemplary embodiments of the invention, and such
exemplifications are not to be construed as limiting the scope of the
invention in any manner.

[0042] As used herein, "proximal" refers to a direction generally toward
the torso of a patient, and "distal" refers to the opposite direction of
proximal, i.e., away from the torso of the patient. As used herein,
"anterior" refers to a direction generally toward the front of a patient.
"Posterior" refers to the opposite direction of anterior, i.e., toward
the back of the patient.

[0043] For purposes of the present disclosure, a sagittal plane is a plane
which extends distally and proximally, as well as anteriorly and
posteriorly. For example, the plane of left/right symmetry in the human
body is a sagittal plane. In the context of a prosthesis, such as
prosthesis 10 described below, the plane that generally divides the
prosthesis into medial and lateral halves is a sagittal plane, and may be
inclusive of an anteroposterior axis such as home axis AH (described
below).

[0044] For purposes of the present disclosure, a transverse plane is
perpendicular to the sagittal plane, and extends medially and laterally
as well as anteriorly and posteriorly. For example, a plane that
separates the human torso from the legs is a transverse plane. In the
context of a prosthesis, the bone-contacting surface (e.g., surface 35
shown in FIG. 1A and described below) and the corresponding proximal
surface of a tibia after resection both define generally transverse
planes. A coronal plane is perpendicular to the sagittal and transverse
planes. For example, the plane separating the front and back sides of a
human is a coronal plane.

[0045] Referring to FIG. 2A, tibia T includes tibial tubercle B having
mediolateral width W, with tubercle midpoint PT located on tubercle
B approximately halfway across width W. While tubercle B is shown as
having midpoint PT at the "peak" or point of maximum anterior
eminence, it is recognized that midpoint PT of tibia T may be spaced
from such a peak. Tibia T also includes attachment point Cp representing
the geometric center of the attachment area between the anatomic
posterior cruciate ligament (PCL) and tibia T. Recognizing that the PCL
typically attaches to a tibia in two ligament "bundles," one of which is
relatively anterior, lateral and proximal and the other of which is
relatively posterior, medial and distal, attachment point CP is
contemplated as representing the anterior/lateral attachment area in an
exemplary embodiment. However, it is contemplated that the
posterior/medial attachment area, or the entire attachment area, could be
used.

[0046] In the context of patient anatomy, "home axis" AH (FIG. 2A)
refers to a generally anteroposterior axis extending from posterior point
Cp to an anterior point CA, in which anterior point CA is
disposed on tubercle B and medially spaced from tubercle midpoint PT
by an amount equal to W/6. Stated another way, anterior point CA is
laterally spaced by an amount equal to W/3 from the medial end of
mediolateral width W, such that point CA lies on the "medial third"
of the anterior tibial tubercle.

[0047] In the context of a prosthesis, such as tibial prosthesis 10
described below, "home axis" AH refers to an axis oriented with
respect to baseplate 12 such that the baseplate home axis AH of
baseplate 12 is aligned with home axis AH of tibia T after
implantation of baseplate 12 in a proper rotational and spatial
orientation. In the illustrative embodiment shown in FIG. 2B and
described in detail below, home axis AH bisects PCL cutout 28 at the
posterior portion of periphery 200 of tibial plate 18 (FIG. 2A), and
bisects anterior edge 202 at the anterior edge of periphery 200 of tibial
plate 18. It is contemplated that home axis AH may be oriented to
other baseplate features, it being understood home axis AH of
baseplate 12 is positioned such that that proper alignment and
orientation of baseplate 12 upon tibia T positions home axis AH of
baseplate 12 coincident with home axis AH of tibia T. Home axis
AH of tibial baseplate 12 may be said to be an anteroposterior axis,
as home axis AH extends generally anteriorly and posteriorly when
baseplate 12 is implanted upon tibia T.

[0048] The embodiments shown and described in the Figures illustrate a
left knee and associated features of a left-knee prosthesis. In an
exemplary embodiment, an associated right knee configuration is a mirror
image of the left-knee configuration about a sagittal plane. Thus, it
will be appreciated that all aspects of the prosthesis described herein
are equally applicable to a left- or right-knee prosthesis.

[0049] 1. Tibial Prosthesis Construction.

[0050] Referring now to FIGS. 1A and 1B, tibial prosthesis 10 includes
tibial baseplate 12 and tibial bearing component 14. Tibial baseplate 12
may include a stem or keel 16 (see, e.g., FIGS. 3A, 3C and 4A) extending
distally from a proximal tibial plate 18 for fixation of tibial baseplate
to a tibia T. Alternatively, a plurality of fixation pegs (not shown) may
be provided to affix tibial plate 18 to tibia T.

[0051] Referring now to FIGS. 1A and 2A, tibial baseplate 12 includes
lateral condylar compartment 20 and medial condylar compartment 22 which
form medial and lateral "halves" of tibial plate 18 divided by home axis
AH (which extends between compartments 20, 22 as shown in FIG. 2B).
However, lateral and medial condylar compartments 20, 22 are dissimilar
in size and shape, rendering tibial plate 18 of tibial baseplate 12
asymmetrical about home axis AH such that medial compartment 22
actually represents more than half of the total area contained within
periphery 200. Periphery 200 represents the outer limits, or bounds, of
lateral and medial compartments 20, 22.

[0053] This asymmetry is specifically designed so that peripheral wall 25
traces the perimeter of the resected proximal surface of tibia T, such
that tibial plate 18 covers a large proportion of the resected proximal
tibial surface as shown in FIG. 2A. This substantial coverage encourages
and facilitates proper rotational and spatial orientation of tibial
baseplate 12 upon tibia T, and provides a large overall profile of
baseplate 12 which creates sufficient space for large-radius,
"soft-tissue friendly" edges as described in detail below. Exemplary
asymmetric profiles for tibial baseplate 12 are described in U.S. patent
application Ser. Nos. 13/189,336, 13/189,338 and 13/189,339, each filed
on Jul. 22, 2011 and entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE
PROSTHESIS, the entire disclosures of which are hereby expressly
incorporated by reference herein.

[0054] As best seen in FIGS. 2A and 2B, lateral condylar compartment 20 of
tibial plate 18 defines overall anteroposterior extent DL which is
less than overall anteroposterior extent DM of medial condylar
compartment 22. This disparity in anteroposterior extent arises from the
additional posterior reach of medial condylar compartment 22 as compared
to lateral condylar compartment 20. The additional posteromedial material
of tibial plate 18 includes chamfer 32 (FIG. 1A) formed in peripheral
wall 25, which forms angle α (FIG. 7) with bone-contacting surface
35 of tibial plate 18. As described in detail below, chamfer 32 forms
part of a larger posteromedial chamfer that provides a relief space for
soft tissues and bone in deep flexion of prosthesis 10.

[0058] Turning to FIG. 2B, periphery 200 of tibial plate 18 surrounds
lateral compartment 20 and medial compartment 22, each of which define a
plurality of lateral and medial arcs extending between anterior edge 202
and lateral and medial posterior edges 204, 206 respectively. In the
illustrative embodiment of FIG. 2B, anterior edge 202, lateral posterior
edge 204 and medial posterior edge 206 are substantially planar and
parallel for ease of reference. However, it is contemplated that edges
202, 204, 206 may take on other shapes and configurations within the
scope of the present disclosure, such as angled or arcuate.

[0059] Generally speaking, a "corner" of periphery 200 may be said to be
that portion of the periphery where a transition from an anterior or
posterior edge to a lateral or medial edge occurs. For example, in the
illustrative embodiment of FIG. 2B, the anterior-lateral corner is
principally occupied by anterior-lateral corner arc 210, which defines a
substantially medial-lateral tangent at the anterior end of arc 210 and a
substantially anteroposterior tangent at the lateral end of arc 210.
Similarly, the anterior-medial corner of periphery 200 is principally
occupied by anterior-medial corner arc 220, which defines a substantially
medial-lateral tangent at the anterior end of arc 220 and a more
anteroposterior tangent at the lateral end of arc 220. Posterior-lateral
arc 214 and posterior-medial arc 224 similarly define substantially
medial-lateral tangents at their respective posterior ends and
substantially anteroposterior tangents at the lateral and medial ends,
respectively.

[0060] As shown in FIGS. 1B and 2A, the outer periphery of tibial bearing
component 14 generally corresponds with the outer periphery 200 of tibial
plate 18, except for the posteromedial extent of plate 18 as compared
with tibial bearing component 14. The anterolateral "corner" of tibial
bearing component 14 defines radius R3 having a generally common
center with radius R1 of baseplate 12 in a transverse plane, i.e.,
radii R1 and R3 are substantially coincident in a plan view.
Similarly, the anteromedial "corner" of tibial bearing component 14
defines radius R4 having a generally common center with radius
R2 of baseplate 12 in a transverse plane, i.e., radii R2 and
R4 are substantially coincident when in a plan view. R3 defines
a slightly smaller radial length as compared to R1, and R4
defines a slightly smaller radial length as compared to R2, such
that the anterior portion of perimeter wall 54 of tibial bearing
component 14 is set back slightly from the anterior portion of peripheral
wall 25 of tibial baseplate 12. As with the above-described comparison
between radii R1 and R2, anteromedial radius R4 is
substantially larger than anterolateral radius R3.

[0061] Medial portion 41 of tibial bearing component 14 may be biased
anteriorly, such that the anterior-medial edges of tibial bearing
component 14 and tibial plate 18 coincide as shown in FIG. 2A. This
anterior bias leaves baseplate chamfer 32 fully exposed at the portion of
tibial plate 18 corresponding to posterior-medial corner 224 and
posterior edge 206 of periphery 200 (FIG. 2B). In contrast, lateral
articular surface 40 substantially completely covers lateral compartment
20 of tibial plate 18, and is generally centered with respect to lateral
compartment 20. In view of this anterior bias of medial portion 41, it
may be said that tibial bearing component 14 is asymmetrically oriented
upon tibial plate 18 such that medial portion 41 appears to have been
rotated forward. In addition to ensuring exposure of baseplate chamfer
32, this asymmetric mounting of tibial bearing component 14 upon tibial
plate 18 ensures a desired articular interaction between tibial
prosthesis 10 and femoral component 60, as described in detail below.

[0062] In the illustrated embodiment, tibial plate 18 includes cutout 28
(FIG. 1A) disposed between condylar compartments 20, 22 to leave PCL
attachment point CP (FIG. 2A) accessible and allow the PCL to pass
therethrough. Tibial bearing component 14 similarly includes cutout 30
(FIG. 1A). Thus, tibial prosthesis 10 is adapted for a cruciate retaining
(CR) surgical procedure, in which the posterior cruciate ligament is not
resected during implantation of tibial prosthesis 10. However, it is
contemplated that a prosthesis in accordance with the present disclosure
may be made for "posterior stabilized" (PS) or "ultracongruent" (UC)
designs in which the posterior cruciate ligament is resected during
surgery. Thus, PCL cutouts 28, 30 may be optionally omitted for
prostheses which do not retain the anatomic PCL. One illustrative PS
design, shown in FIG. 3C, includes proximally extending spine 45
monolithically formed with tibial bearing component 14C. Spine 45 is
designed to interact with a corresponding cam (not shown) of a femoral
component (e.g., femoral component 60 shown in FIG. 7).

[0063] In an alternative embodiment, tibial baseplate 12 may be omitted
such that tibial prosthesis 10 is formed solely from tibial bearing
component 14. Tibial bearing component 14 may have a stem or keel (not
shown) similar to keel 16 of baseplate 10, or may have fixation pegs for
fixation to tibia T. Tibial bearing component 14 may therefore have
lateral and medial portions 39, 41 and a distal fixation structure which
are monolithically formed of a single material, such as polyethylene or
another suitable polymer. Alternatively, lateral and medial portions 39,
41 may be made of a different, but integrally formed material as compared
to the distal fixation structure.

[0064] Advantageously, the relatively large area of bone contacting
surface 35 of tibial plate 18 facilitates a large amount of bone ingrowth
where bone ingrowth material is provided in tibial baseplate 12. For
example, baseplate 12 may be at least partially coated with a highly
porous biomaterial to facilitate firm fixation thereof to tibia T. A
highly porous biomaterial is useful as a bone substitute and as cell and
tissue receptive material. A highly porous biomaterial may have a
porosity as low as 55%, 65%, or 75% or as high as 80%, 85%, or 90%. An
example of such a material is produced using Trabecular Metal®
Technology generally available from Zimmer, Inc., of Warsaw, Ind.
Trabecular Metal® is a trademark of Zimmer, Inc. Such a material may
be formed from a reticulated vitreous carbon foam substrate which is
infiltrated and coated with a biocompatible metal, such as tantalum, by a
chemical vapor deposition ("CVD") process in the manner disclosed in
detail in U.S. Pat. No. 5,282,861 to Kaplan, the entire disclosure of
which is expressly incorporated herein by reference. In addition to
tantalum, other metals such as niobium, or alloys of tantalum and niobium
with one another or with other metals may also be used.

[0065] Generally, the porous tantalum structure includes a large plurality
of struts (sometimes referred to as ligaments) defining open spaces
therebetween, with each strut generally including a carbon core covered
by a thin film of metal such as tantalum, for example. The open spaces
between the struts form a matrix of continuous channels having no dead
ends, such that growth of cancellous bone through the porous tantalum
structure is uninhibited. The porous tantalum may include up to 75%, 85%,
or more void space therein. Thus, porous tantalum is a lightweight,
strong porous structure which is substantially uniform and consistent in
composition, and closely resembles the structure of natural cancellous
bone, thereby providing a matrix into which cancellous bone may grow to
provide fixation of implant 10 to the patient's bone.

[0066] The porous tantalum structure may be made in a variety of densities
in order to selectively tailor the structure for particular applications.
In particular, as discussed in the above-incorporated U.S. Pat. No.
5,282,861, the porous tantalum may be fabricated to virtually any desired
porosity and pore size, and can thus be matched with the surrounding
natural bone in order to provide an improved matrix for bone ingrowth and
mineralization.

[0067] 2. Soft Tissue Impact Reduction and Deep Flexion Enablement.

[0068] Tibial bearing component 14 advantageously reduces the potential
impact of prosthesis 10 on the adjacent anatomic soft tissues of a knee
after implantation, even when the prosthesis is articulated into deep
flexion in vivo. This reduced impact results from features included in
bearing component 14, and such features are facilitated by the size,
shape and configuration of tibial baseplate 12.

[0070] As best seen in FIGS. 1A and 7, baseplate chamfer 32 extends
proximally and anteriorly from a posterior/distal edge 62, corresponding
to posterior edge 206 of periphery 200 shown in FIG. 2B, to an
anterior/proximal edge 64 of chamfer 32. Similarly, bearing chamfer 50
extends proximally and anteriorly from posterior/distal edge 66, which is
coincident with inferior surface 36 of bearing component 14, to an
anterior/proximal edge 68 at the boundary of medial articular surface 42.
When tibial bearing component 14 is assembled to tibial baseplate 12 as
shown in FIGS. 1B and 7, medial portion 41 of tibial bearing component 14
(described above) is positioned to substantially align chamfers 32, 50.
When so aligned, posterior/distal edge 66 of bearing chamfer 50 is
disposed near anterior/proximal edge 64 of baseplate chamfer 32, such
that chamfers 32, 50 cooperate to define a substantially continuous
chamfer extending from the resected surface of tibia T to medial
articular surface 42. However, as noted below, it is also contemplated
that tibial and baseplate chamfers can cooperate to define a
discontinuous chamfer within the scope of the present disclosure

[0071] Chamfers 32, 50 cooperate to define relief 52 (FIG. 7) formed
between femur F and tibial plate 18 when tibial prosthesis 10 is in a
deep flexion orientation. In the illustrated embodiment of FIG. 7, the
deep flexion orientation is defined by angle β between anatomic
tibia axis AT and anatomic femoral axis AF of up to about 25
degrees to about 40 degrees, for example (i.e., about 140 degrees to 155
degrees of flexion or more).

[0072] Although asymmetric periphery 200 is designed to closely match an
anatomic resected tibial surface as described above, certain aspects of
periphery 200 are designed to intentionally deviate from the calculated
anatomical shape to confer particular advantages with regard to
minimization of soft tissue impact and the associated implanted knee
prosthesis. Referring to FIG. 2A, for example, posterior edge 206 (FIG.
2B) of medial compartment 22 may be "pulled back" from the adjacent
posterior-medial edge of tibia T to define void 58. In an exemplary
embodiment, void 58 is created by leaving about 4 mm (measured as an
anteroposterior extent) of the proximal resected surface of tibia T
exposed. However, it is contemplated that void 58 may be smaller, or may
be nonexistent. For some patient anatomies, for example, it may be
possible to maximize tibial coverage by eliminating void 58 entirely
(i.e., by positioning posterior edge 206 of medial compartment flush with
the corresponding edge of tibia T). A surgeon may choose to eliminate gap
58 when presented with the opportunity to do so, provided other portions
of periphery 200 do not extend beyond the periphery of the resected
proximal surface of tibia T.

[0073] As illustrated in FIG. 7, void 58 cooperates with chamfers 32, 50
to create extra space for adjacent soft tissues and bone, particularly
when prosthesis 10 is in a deep flexion configuration as illustrated.
Advantageously, relief 52 between femur F and tibia T, made possible by
chamfers 32, 50 as described above, cooperates with the "pulled back" or
incongruent posterior medial void 58 to allow the deep flexion
orientation to be achieved without allowing soft tissues to become
trapped and impinged between femoral component 60/femur F and tibial
plate 18/tibial bearing component 14. In deep flexion, soft tissues in
the region of relief 52 can shift slightly into void 58 between femur F
and tibia T within minimal resistance, thereby mitigating soft-tissue
impacts by decreasing the likelihood of, e.g., compression or impingement
with surrounding components. Moreover, any contact that may occur between
a prosthesis made in accordance with the present disclosure and adjacent
soft tissues will occur against the flat and broadly rounded surfaces of
the prosthesis, such the impact of such contact on the tissue is
minimized. To this end, it is contemplated that the specific geometry of
chamfers 32, 50 may be modified for individual patient needs, such as to
accommodate abnormally positioned and/or sized adjacent soft tissues.

[0074] In the illustrated embodiment of FIG. 7, bearing chamfer 50 defines
a substantially linear profile in a sagittal plane. Where this linear
profile extends across the medial/lateral extent of chamfers 32, 50,
chamfers 32, 50 will also be generally coplanar, as illustrated. Chamfers
define angle α with a transverse plane, e.g., with superior surface
34, bone contacting surface 35 and/or inferior surface 36 (which all lie
in a generally transverse plane in the illustrated embodiment). Angle
α is an acute angle that may have a value as little as about 35
degrees or 50 degrees and as large as about 55 degrees, 61 degrees, 70
degrees or 75 degrees, or within any range defined by any of the
foregoing values.

[0075] Lateral chamfer 51 (FIG. 1A) may also be provided in order to
facilitate a smooth transition from lateral articular surface 40 to the
interface between inferior surface 36 of tibial bearing component 14 and
superior surface 34 of tibial baseplate 12. Because lateral compartment
20 of periphery 200 (FIGS. 2A and 2B) provides maximum coverage of the
resected proximal surface of tibia T, not all of the substantially
conforming lateral portion 39 of tibial bearing 14 is needed to form
lateral articular surface 40. Lateral chamfer 51 occupies the marginal
space near articular surface 40, which is not normally used in
articulation of prosthesis 10 with femoral component 60 (FIG. 7).

[0076] Advantageously, the smooth, rounded transition provided by lateral
chamfer 51 provides clearance for bone and tissue during flexion. If an
adjacent soft tissue does come into contact with lateral chamfer 51, the
tension arising from such contact will be lower as compared to a
prosthesis lacking such chamfer. Moreover, as with the other chamfers and
rounded profiles provided on prosthesis 10, the rounded transition of
lateral chamfer 51 minimizes the impact caused by any contact which may
occur between chamfer 51 and adjacent soft tissues. At the same time, the
buildup of material around lateral chamfer 51 provides posterior
constraint to femoral component 60 (FIG. 7) and strengthens the posterior
portion of lateral portion 39 of bearing component 14.

[0077] It is contemplated that bearing chamfer 50 may have an arcuate
profile in a sagittal, coronal and/or transverse plane, and may include
convex or concave curvature as required or desired for a particular
application. For example, bearing component 14A shown in FIG. 3A is
similar to bearing component 14 described above, and reference numbers in
FIGS. 3A and 3B refer to analogous structures shown in FIGS. 1A and 2A
and described above with respect to bearing component 14. However,
chamfer 50A defines a slight curve in a sagittal plane as chamfer 50A
extends from anterior/proximal edge 68A toward posterior/distal edge 66A
of tibial bearing component 14A. For purposes of evaluating angle α
(FIG. 7) in the context of curved chamfer 50A, a sagittal tangent line
drawn at anterior/proximal edge 68A and compared to a coronal plane as
described above. In the exemplary illustrated embodiment, angle α
is about 61 degrees.

[0078] In the context of chamfers, e.g. chamfers 32, 50 and 50A, chamfer
edges are referred to herein as "anterior/proximal" and
"posterior/distal." These references refer to the relative positions of
the chamfer edges in the context of the chamfers themselves, in the
context of the position and orientation of the tibial prosthesis after
implantation. Thus, an "anterior/proximal" edge is located at or near the
anterior and proximal terminus of the chamfer, while a
"posterior/proximal" edge located at or near the posterior and distal
terminus of the chamfer (i.e., at the opposite end of the chamfer).

[0079] In the illustrative embodiment of FIG. 3A, chamfer 50A spans
substantially the entire available proximal/distal distance, i.e., from
superior surface 38A to anterior/proximal edge 64 of baseplate chamfer
32. However, it is contemplated that a chamfer in accordance with the
present disclosure may extend across only a part proximal/distal
distance, beginning near superior surface 38A but ending at a location
proximal of the junction between the tibial plate (e.g., plate 18) and
the tibial bearing component (e.g., component 14). After "early" terminus
of the chamfer, the remainder of the vertical distance may be taken up by
a vertical section of the bearing component periphery.

[0080] For example, chamfer 50A may extend as little as 25% or 32% of the
total available proximal/distal distance, or as much as 100% of the total
available proximal/distal distance, or may span and percentage distance
within any range defined by any of the foregoing values. Moreover, it is
contemplated that the configuration of chamfer 50A may vary depending on
the configuration of tibial bearing component 14A. Where bearing
component 14A is relatively thin, such as about 9-10 mm, for example,
chamfer 50A may extend across a relatively larger proportion of the total
available proximal/distal distance.

[0081] In some instances, bearing component 14A may be made thicker to
accommodate additional resection of tibia T. For example, one such
thicker bearing component is illustrated as component 14B, shown FIG. 4A
and discussed below. In these instances, a similar chamfer to chamfer 50A
may be provided in the proximal 9-10 mm of the available proximal/distal
distance, while the remainder of such distance may be substantially
vertical. This chamfer configuration retains the advantages provided by
chamfer 50A, such as avoidance of soft tissue and bone impingement in
deep flexion. Thus, in a relatively thicker component, a relatively
smaller proportion of the total available proximal/distal distance may be
needed to create a chamfer which provides the benefits described herein.
In an exemplary embodiment, tibial bearing component may be provided as a
kit of increasing thicknesses, with each thickness growing incrementally
larger by 0.5 mm-1.0 mm. It is contemplated that such incremental growth
in thickness may be larger, such as 2 mm, 3 mm or 4 mm for example.

[0082] In some other instances, the distal bone stock of femur F (FIG. 7)
is significantly resected and tibial bearing component 14A is made
thicker to accommodate the resulting proximal/distal joint space occupied
by prosthesis 10. In these instances, a large proportion of the thicker
bearing component may be given over to chamfer 50A, such that chamfer 50A
extends across a large proportion of the available proximal/distal
distance. This extensive chamfer 50A will advantageously minimize the
chances for impingement of the adjacent soft tissues and bone.

[0083] The slight sagittal curve of chamfer 50A (described above) defines
a sagittal chamfer radius RC1 (FIG. 3A) which may be between as
little as 5 mm or 65 mm and as much as 75 mm or 180 mm, or may be any
value within any range defined by any of the foregoing values. Radius
RC1 extends across an anteroposterior extent DCA of about 2.0
mm, such that the length of the arc defined by radius RC1 is about 4
mm where angle α is 61 degrees (as noted above). However, it is
contemplated that anteroposterior extent DCA may be between about
0.5 mm and about 10.0 mm, and the arc length may vary accordingly.

[0084] A second radius, shown as radius RC2 in FIG. 3A, is tangent to
the posterior/distal end of radius RC1 and spans the remaining
distance to posterior/distal edge 66A to complete chamfer 50A. Radius
RC2 is smaller than radius RC1, and may have a value as little
as 5 mm or 12.5 mm and as much as 12.8 mm or 180 mm, or may be any value
within any range defined by any of the foregoing values. Radius RC1
cooperates with radius RC2 to span the entire anteroposterior extent
DCP of chamfer 50A, which extends from anterior/proximal edge 68A to
posterior/distal edge 66A of bearing chamfer 50A as noted above.
Anteroposterior extent DCP ranges from about 0.5 mm to about 10.0
mm. In the illustrated exemplary embodiment of FIG. 3B, anteroposterior
extent DCP is about 2.7 mm. Thus, in the illustrated embodiment, the
anteroposterior extent of radius RC2 is about 0.7 mm.

[0085] The particular arrangement of chamfer 50A, as described above, has
been found to represent an excellent balance between competing interests.
On one hand, soft-tissue clearance is maximized by decreasing angle
α, which increases the volume available in void 58. On the other
hand, the additional material afforded by increasing angle α at the
posteromedial portion of bearing component serves as a strengthening
buttress, thereby providing a more robust bearing component. Chamfer 50A
represents a strong component geometry that also provides enough space
for natural soft tissues across a wide range of expected anatomical
variability among patients.

[0086] However, it is contemplated that other chamfer profiles may be
utilized within the scope of the present disclosure. Such profiles may
include, for example, multiple linear sections cooperating to approximate
a rounded profile, a pair of linear sections, or a concave rounded
profile. Moreover, it is contemplated that patient-specific chamfer
profiles may be created to match the anatomies of individual patients.
For a patient-specific design, the posteromedial chamfer may be designed
to correspond to the sagittal profile of the portion of the femur which
is adjacent the posteromedial chamfer in deep flexion of the knee.

[0087] In an exemplary embodiment, a kit of prostheses may be provided
with bearing components that all share common geometrical features of
chamfer 50A. Referring to FIG. 3A, for example, bearing component 14A
defines distance DC from the anterior edge thereof to
anterior/proximal edge 68A of bearing chamfer 50A. In a kit of different
prosthesis sizes designed to accommodate patients having various bone
sizes, distance DC may vary widely. For example, distance DC
may be as little as 20 mm, 25 mm or 36 mm for small prosthesis sizes, and
as much as 56 mm, 65 mm, or 75 mm for large prosthesis sizes, or may be
any value within any range defined by any of the foregoing values.

[0088] Despite this substantial variability, exemplary bearing components
(including component 14A) can utilize a common angle α,
anteroposterior extent DCA of the proximal/anterior portion of the
chamfer, and overall chamfer anteroposterior extent DCP as described
above. However, it is contemplated that radii RC1, RC2 may vary
across prosthesis sizes, such as within the ranges set forth above, in
order to ensure smooth and "soft-tissue friendly" transitions from the
medial articular surface (e.g., surface 42) to the chamfer (e.g., chamfer
50A).

[0089] Turning to FIGS. 4A and 4B, prosthesis 10B including a thickened
tibial bearing component 14B is shown. Bearing component 14B shown in
FIG. 4A is similar to bearing component 14A described above, and
reference numbers in FIGS. 4A and 4B refer to analogous structures shown
in FIGS. 3A and 3B and described above with respect to bearing component
14A. However, bearing component 14B defines overall thickness TB
which is substantially larger than the corresponding overall thickness
TA of bearing component 14A. Bearing component 14B defines the same
overall anteroposterior extent as bearing component 14A (i.e., distance
DC plus distance DCP), and may be used interchangeably with
bearing component 14A to effectively increase the overall thickness of
prosthesis 10. Such increased thickness may be used to accommodate a more
extensive resection of tibia T, as noted above, or to accommodate the
ligament configuration of a particular patient, for example.

[0090] Thickened bearing component 14B includes bearing chamfer 50B, which
spans substantially the entire distance in a sagittal plane, as shown,
from anterior/proximal edge 68B to posterior/distal edge 66B. Despite
this additional proximal/distal span of chamfer 50B as compared to
chamfer 50A, anteroposterior extents DCA and DCP remain
unchanged, i.e., at about 2.0 mm and about 2.7 mm respectively. Angle
α, again taken from a tangent to the arcuate sagittal profile of
the proximal portion of chamfer 50B, also remains unchanged.

[0091] Radius RC3, which remains the radius value for chamfer 50B
across the anteroposterior extent DCA in a similar fashion to Radius
RC1 discussed above, is larger than radius RC4 which extends
across the remainder of overall anteroposterior extent DCP in
similar fashion to radius RC2. However, the nominal values of radii
RC3, RC4 may be different from radii RC1, RC2
respectively. In an exemplary embodiment, for example, radius RC3
may have a value as little as 55 mm or 65 mm and as much as 75 mm or 180
mm, or may be any value within any range defined by any of the foregoing
values. Radius RC4 may have a value as little as 5 mm or 12.5 mm and
as much as 12.8 mm or 180 mm, or may be any value within any range
defined by any of the foregoing values.

[0092] Advantageously, chamfer 50B defines a chamfer profile that is
substantially the same as chamfer 50A near anterior/proximal edge 68B,
thereby preventing impingement of femur F and/or adjacent soft tissues in
a similar manner to chamfer 50A. Meanwhile, the reduction in radius
RC3 as compared to radius RC1, imparts an overall "steeper"
sagittal profile to chamfer 50B as compared to chamfer 50A. This steeper
profile provides additional posterior buttressing of medial portion 41A,
while the additional thickness TB provides for ample volume in void
58 for soft tissue clearance.

[0093] In addition to the posteromedial features discussed above,
additional soft-tissue impact reduction may be achieved at the medial and
lateral edges of bearing component 14. The relatively large size of
tibial plate 18 (covering a large proportion of the resected proximal
surface of tibia T) cooperates with the close congruence of tibial
bearing component 14 thereto to enable a relatively large superior
surface 38 of tibial bearing component 14. Because not all of this large
superior surface area 38 is needed for lateral and medial articular
surfaces 40, 42 (FIG. 2A), tibial bearing component 14 provides
sufficient non-articular surface area around the periphery of lateral and
medial articular surfaces 40, 42 to allow for "pulled back" areas and
relatively large-radius, rounded transitions between such articular
surfaces and peripheral wall 54 of tibial bearing component 14. These
features minimize or prevent friction between tibial prosthesis 10 and
any surrounding soft tissues, such as the iliotibial (IT) band, which may
remain in place after implantation of prosthesis 10.

[0094] Similar to the "pulled back" portion of periphery 200 in the
posteromedial portion at posterior-medial corner 224 and posterior edge
206, described in detail above, tibial baseplate 12 and tibial bearing
component 14 each have anterior-lateral corners which are intentionally
"pulled back" from an expected periphery of tibia T to create gap 56
(FIG. 2A) between the anterior-lateral area of the resected surface of
tibia T and prosthesis 10. Advantageously, gap 56 moves the
anterior-lateral corners of baseplate 12 and tibial bearing component 14
away from the area typically occupied by the iliotibial band, thereby
minimizing the potential for impingement of the IT band upon prosthesis
10. In an exemplary embodiment, gap 56 may range from 0.5 mm for a
small-size prosthesis, to 1 mm for a medium-sized prosthesis, to 2 mm for
a large-sized prosthesis.

[0095] For certain patients or in certain ranges of prosthesis
articulation, however, the human iliotibial (IT) band may touch the
anterolateral corner of prosthesis 10. In some instances, the medial
collateral ligament (MCL) may also touch the medial edge of prosthesis
10. As noted above, the large available surface area afforded by
asymmetric periphery 200 of tibial baseplate 12 also affords ample space
for peripheral transitions from superior surface 38 to peripheral wall 54
of tibial bearing component 14.

[0096] Turning to FIGS. 3C and 3D, transition radii RTL, RTM are
illustrated as the radii formed by the transition between lateral and
medial articular surfaces 40, 42 and lateral and medial edges 72, 74 of
peripheral wall 54 respectively. As best seen in FIG. 3D with respect to
the medial side of prosthesis 10C, the ample margin between the outer
limits of medial articular surface 42 and medial edge 74 of peripheral
wall 54 allows medial transition radius RTM to be relatively large,
thereby allowing such transition to define a relatively large convex
profile at lateral edge 72 and medial edge 74 of peripheral wall 54 while
still leaving sufficient concave space, constraint and conformity for
articular surfaces 40, 42. Lateral transition radius RTL similarly
occupies a large margin between the outer limits of lateral articular
surface 40 and lateral edge 72 of peripheral wall 54, though the margin
is slightly smaller. Therefore, lateral transition radius RTL may be
slightly less than medial transition radius RTM.

[0097] In an exemplary embodiment, medial transition radius RTM is at
least zero mm or 0.45 mm and may be as large as 3 mm, 5 mm or 7 mm, or
may be any value within any range defined by any of the foregoing values.
Lateral transition radius RTL is at least zero mm or 0.5 mm and may
be as large as 2 mm, 5 mm or 7 mm, or may be any value within any range
defined by any of the foregoing values.

[0098] In addition to radii RTM, RTL the respective transitions
from lateral and medial articular surfaces 40, 42 to lateral and medial
edges 72, 74 may also be expressed with reference to the arc length
defined by radii RTM, RTL. Moreover, a longer arc length
results in an increasingly broad, convex lateral and medial transition,
which in turn provides a large contact area for soft tissue. For example,
if an adjacent soft tissue structure (e.g., the IT band or medial
collateral ligament) comes into contact with tibial bearing component 14,
minimal contact pressures therebetween are experienced if large arc
lengths are provided. In an exemplary embodiment, the medial arc length
may be as little as 0 mm or 0.83 mm and may be as large as 6.4 mm, or may
be any value within any range defined by any of the foregoing values.
Lateral arc length may be as little as zero mm or 0.9 mm and may be as
large as 3.5 mm or 6.4 mm, or may be any value within any range defined
by any of the foregoing values.

[0099] Further, the anterolateral "pull back" of the anterior-lateral
corner of prosthesis 10, described above, allows the corresponding
anterior-lateral corner of bearing component 14 to maintain separation
from the IT band through a wide range of flexion, such that only very low
contact pressures are present in the limited circumstances where contact
may occur.

[0100] Prosthesis 10C shown in FIG. 3C is similar to prostheses 10, 10A,
10B described above, and reference numbers in FIGS. 3C and 3D refer to
analogous structures shown in FIGS. 1A through 3B, 4A and 4B and
described above with respect to prostheses 10, 10A and 10B. Moreover, it
should be appreciated that the features described herein with respect to
any of prostheses 10, 10A, 10B may be applied to each of the prostheses
described herein.

[0101] For example, in the illustrative embodiment of FIG. 3C, tibial
bearing component 14C includes spine 45 extending proximally from
superior surface 38 rather than eminence 44. As noted above, spine 45 is
appropriate for use in a posterior-stabilized (PS) prosthesis. Large
transition radii RT may be provided on PS designs as shown, or on CR
designs.

[0102] Tibial prosthesis 10 (inclusive of tibial prostheses 10A, 10B and
10C) can be considered "soft tissue friendly" because the edges of tibial
bearing component 14 and tibial plate 18, including chamfers 32, 50, are
smooth and rounded, so that soft tissue coming into contact with these
edges will be less likely to chafe or abrade. Further, the high
congruence peripheral wall 54 of bearing component 14 and peripheral wall
25 of baseplate 12 provides coverage of nearly all of superior surface 34
of baseplate 12 with bearing component 14, thereby preventing contact
between any soft tissue and any metal edge of baseplate 12. Instead,
where contact does occur, it is with the soft, polymeric edges of tibial
bearing 14 or with the flat or gently convex surfaces of chamfers 32, 50.

[0103] 3. Trial Tibial Prostheses

[0104] As noted above, a kit of tibial prosthesis 10 may be provided with
a variety of sizes and configurations to accommodate different bone sizes
and geometries. The choice of one particular size may be planned
preoperatively such as through preoperative imaging and other planning
procedures. Alternatively, an implant size may be chosen, or a previous
size choice modified, intraoperatively. To facilitate proper
intraoperative selection of a particular size for tibial prosthesis 10
from among a range of available sizes, and to promote proper orientation
of the chosen prosthesis 10, tibial prosthesis 10 may be part of a kit
including one or more template or "trial" components.

[0105] Referring now to FIGS. 5 and 6, trial prosthesis 100 may be
temporarily coupled to tibia T for intraoperative sizing evaluation of
tibial prosthesis 10 and initial steps in the implantation of tibial
prosthesis 10. Trial prosthesis 100 is one of a set of trial prostheses
provided as a kit, with each trial prosthesis having a different size and
geometrical configuration. Each trial prosthesis in the set of trial
prostheses corresponds to one among several sizes of permanent prosthesis
10, such as to varying peripheries 200 of tibial baseplate 12 as
described above.

[0106] For example, as shown in FIG. 5, trial prosthesis 100 defines
superior surface 112 generally corresponding in size and shape to
periphery 200 of tibial plate 18, and including lateral portion 102 and
medial portion 104. Like periphery 200, superior surface 112 is
asymmetrical about home axis AH, with lateral portion 102 having a
generally shorter overall anteroposterior extent as compared to medial
portion 104 (in part because medial portion 104 includes void indicator
106 as discussed below). In addition, the anterolateral "corner" of
lateral portion 102 defines radius R1, which is identical to radius
R1 of periphery 200, while the anteromedial "corner" of medial
portion 104 defines radius R2, which is identical to radius R2
of periphery 200 and is therefore greater than radius R1.

[0107] Moreover, trial prosthesis 100 includes perimeter wall 114 which
defines a substantially identical periphery as peripheral wall 25 of
tibial plate 18, and therefore has the same geometrical features and
shapes of periphery 200 described above with respect to tibial plate 18.
Thus, the nature of the asymmetry of trial prosthesis 100 changes across
the various sizes of tibial prosthesis provided in the kit including
trial prosthesis 100.

[0108] In an alternative embodiment, a trial prosthesis may be provided
which is designed to extend completely to the posterior-medial edge of
the natural tibial resection periphery. Thus, such a trial would
substantially completely cover the resected tibial surface, thereby
aiding in determination of a proper rotational orientation of the trial
(and, therefore, of the final tibial baseplate 12). In this alternative
embodiment, the trial prosthesis lacks the posterior-medial "pull back"
of tibial plate 18, described above, and therefore does not define void
58.

[0110] Void indicator 106 advantageously facilitates proper rotational and
spatial orientation of trial prosthesis 100 on the resected proximal
surface of tibia T by allowing a surgeon to visually match tibial bearing
component 14 with trial prosthesis 100, as described in detail below. In
the illustrated embodiment, void indicator 106 is an area of visual
and/or tactile contrast with the remainder of tibial plate 18. This
contrast may include, for example, a contrasting color, texture, surface
finish, or the like, or may be formed by a geometric discrepancy such as
a step or lip, for example.

[0111] Referring specifically to FIG. 3, trial prosthesis 100 further
includes a plurality of peg hole locators 108 corresponding to the proper
location for peg holes in tibia T to receive pegs (not shown) extending
inferiorly from tibial plate 18 of tibial baseplate 12. Advantageously,
peg hole locators 108 allow a surgeon to demarcate the appropriate
location on the resected proximal surface of tibia T for peg hole centers
after the proper size and orientation for trial prosthesis 100 has been
found. The marked peg hole centers facilitating eventual drilling of
properly located peg holes in tibia T after trial prosthesis has been
removed, as discussed in detail below. Alternatively, peg hole locators
108 may be used as drill guides to drill appropriately positioned peg
holes while trial prosthesis 100 is still positioned on tibia T. As an
alternative to peg hole locators 108, it is contemplated that a central
aperture may be provided as a keel or stem locator for demarcating the
proper location of keel 16 (FIG. 3A).

[0112] Void indicator 106 may also be used to demarcate the implanted
position and location of a baseplate which is symmetric, or has any other
periphery which is different from periphery 200. In some instances, for
example, it may be desirable to use a tibial baseplate different from
baseplate 12. However, the advantages conferred by the asymmetric
periphery of baseplate 12, such as proper rotational orientation and
positioning, may still be realized. Asymmetric trial prosthesis 100 may
be used to locate the proper location for peg holes or a keel, as
discussed herein, with void indicator 106 offering a visual indication of
which part of the resected proximal surface of tibia T will not be
covered over by the differently-shaped tibial baseplate. When the tibial
baseplate is implanted, it will have the same advantageous
rotation/location as baseplate 12 even if the differently-shaped
baseplate covers less bone. The surgeon will also be assured that those
areas of bone not covered by the differently-shaped prosthesis are
proper, having previously seen such areas covered by void indicator 106.

[0113] 4. Tibial Prosthesis Implantation

[0114] In use, a surgeon first performs a resection of tibia T using
conventional procedures and tools, as are well-known in the art.
Exemplary surgical procedures and associated surgical instruments are
disclosed in "Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique,"
"NEXGEN COMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed
Bearing Knee" and "Zimmer NexGen Complete Knee Solution
Extramedullary/Intramedullary Tibial Resector, Surgical Technique"
(collectively, the "Zimmer Surgical Techniques"), copies of which are
submitted on even date herewith, the entire disclosures of which are
hereby expressly incorporated by reference herein.

[0115] In an exemplary embodiment, a surgeon will resect the proximal
tibia to leave a planar surface prepared for receipt of a tibial
baseplate. For example, the surgeon may wish to perform a resection
resulting in a tibial slope defined by the resected tibial surface, which
typically slopes proximally from posterior to anterior (i.e., the
resected surface runs "uphill" from posterior to anterior).
Alternatively, the surgeon may instead opt for zero tibial slope. Varus
or valgus slopes may also be employed, in which the resected surface
slopes proximally or distally from medial to lateral. The choice of a
tibial and/or varus/valgus slope, and the amount or angle of such slopes,
may depend upon a variety of factors including correction of deformities,
mimicry of the native/preoperative tibial slope, and the like.

[0116] Tibial baseplate 12 is appropriate for use with a tibial slope of
as little as zero degrees and as much as 9 degrees, and with a varus or
valgus slope of up to 3 degrees. However, it is contemplated that a
tibial baseplate made in accordance with the present disclosure may be
used with any combination of tibial and/or varus/valgus slopes, such as
by changing the angular configuration of keel 16 with respect to
bone-contacting surface 35.

[0117] With a properly resected proximal tibial surface, the surgeon
selects trial prosthesis 100 from a kit of trial prostheses, with each
prosthesis in the kit having a different size and geometrical
configuration (as discussed above). Trial prosthesis 100 is overlaid on
the resected surface of tibia T. If trial prosthesis 100 is appropriately
sized, a small buffer zone 110 (FIG. 5) of exposed bone of resected tibia
T will be visible around the periphery of trial prosthesis 100. Buffer
zone 110 should be large enough to allow a surgeon to rotate and/or
reposition trial prosthesis 100 within a small range, thereby offering
the surgeon some flexibility in the final positioning and kinematic
profile of tibial prosthesis 10. However, buffer 110 should be small
enough to prevent trial prosthesis 100 from being rotated or moved to an
improper location or orientation, or from being implanted in such as way
as to produce excessive overhang of the edge of trial prosthesis 100 past
the periphery of the resected tibial surface. In one exemplary
embodiment, for example, buffer zone 110 will be deemed to be appropriate
when trial prosthesis 100 can be rotated from a centered orientation by
up to +/-5 degrees (i.e., in either direction). In other embodiments, it
is contemplated that such rotation may be as much as +/-10 degrees or
+/-15 degrees. In still other embodiments, trial prosthesis 100 may
substantially completely match the proximal resected surface of tibia T,
such that buffer zone 110 is eliminated and no rotational freedom is
afforded.

[0118] To aid the surgeon in finding proper rotational orientation, trial
prosthesis 100 may include anterior and posterior alignment indicia 70A,
70P (FIG. 5). Similarly positioned marks may be provided on tibial plate
18 for reference upon final implantation thereof The surgeon can align
anterior indicium 70A with anterior point CA and posterior indicium
70P with PCL attachment point CP, thereby ensuring the anatomical
and component home axes AH (described above) are properly aligned.
Alternatively, a surgeon may use indicia 70A, 70P to indicate a desired
deviance from alignment with home axis AH. As noted above, deviation
of up to 5 degrees is envisioned with the exemplary embodiments described
herein. A surgeon may choose to orient indicia 70A, 70P to another tibial
landmark, such as the middle of the patella or the medial end of tibial
tubercle B.

[0119] The large coverage of trial prosthesis 100 (and, concomitantly, of
tibial plate 18) ensures that tibial baseplate 12 will be properly
positioned and oriented on tibia T upon implantation, thereby ensuring
proper kinematic interaction between tibial prosthesis 10 and femoral
component 60. If buffer zone 110 is either nonexistent or too large,
another trial prosthesis 100 may be selected from the kit and compared in
a similar fashion. This process is repeated iteratively until the surgeon
has a proper fit, such as the fit illustrated in FIGS. 3 and 4, between
trial prosthesis 100 and the proximal resected surface of tibia T.

[0120] With the proper size for trial prosthesis 100 selected and its
orientation on tibia T settled, trial prosthesis 100 is secured to tibia
T, such as by pins, screws, temporary adhesive, or any other conventional
attachment methods. Once trial prosthesis 100 is so secured, other trial
components, such as trial femoral components and trial tibial bearing
components (not shown) may be positioned and used to articulate the leg
through a range of motion to ensure a desired kinematic profile. During
such articulation, void indicator 106 may be used to indicate to the
surgeon that any impingement of femoral component 60, femur F or adjacent
soft tissues upon trial prosthesis 100 at void indicator 106 will not
occur when tibial prosthesis 10 is implanted. Once the surgeon is
satisfied with the location, orientation and kinematic profile of trial
prosthesis 100, peg hole locators 108 may be used to demarcate the
appropriate location of peg holes in tibia T for tibial baseplate 12.
Such peg holes may be drilled in tibia T with trial prosthesis 100
attached, or trial prosthesis 100 may be removed prior to drilling the
holes.

[0121] With tibia T thus prepared for receipt of tibial prosthesis 10,
tibial baseplate 12 may be provided by the surgeon (e.g., procured from a
kit or surgical inventory), and implanted on tibia T, such that implant
pegs (not shown) fit into holes previously identified and created using
peg hole locators 108 of trial prosthesis 100. Tibial baseplate 12 may be
selected from a family or kit of tibial baseplate sizes to correspond
with the chosen size and/or configuration of trial component 100, thereby
ensuring that tibial plate 18 will cover a large proportion of the
resected proximal surface of tibia T, as trial prosthesis 100 did prior
to removal.

[0122] In an alternative embodiment, the surgeon may provide a tibial
baseplate (not shown) having a periphery that does not match periphery
200 of trial prosthesis 100. For example, the surgeon may choose a
baseplate which is symmetric about an anteroposterior axis. In another
example, a surgeon may choose a tibial baseplate having the same
periphery as tibial bearing component 14, and having a vertical
peripheral wall in place of chamfer 32. In this embodiment, void
indicator may be configured to show the non-acuity between periphery 200
and the differently-shaped tibial baseplate, as described above. Upon
implantation of the differently-shaped tibial baseplate, the surgeon can
visually verify that the portions of bone previously covered by void
indicator are not covered by the tibial baseplate

[0123] Tibial baseplate 12 (or an alternative baseplate, as described
above) is implanted upon the proximal surface of tibia T in accordance
with accepted surgical procedures. Exemplary surgical procedures and
associated surgical instruments are disclosed in the Zimmer Surgical
Techniques, incorporated by reference above. Tibial baseplate 12 is
affixed to tibia T by any suitable method, such as by keel 16 (FIGS. 3A,
3C and 4A), adhesive, bone-ingrowth material, and the like.

[0127] Bearing component 14 is not movable with respect to baseplate 12
after the components have been locked to one another, which is to say the
embodiments of prosthesis 10 illustrated herein are "fixed bearing"
designs. Thus, proper location and rotational orientation of tibial
bearing component 14 upon tibial plate 18 is ensured by cooperation
between raised perimeter 24 and peripheral recess 46, as well as by
locking mechanism 26 cooperating with central recess 47. Such proper
orientation results in medial articular surface 42 being generally
anteriorly disposed with respect to medial compartment 22 of tibial plate
18, as noted above. It is also contemplated that the principles of the
present disclosure may be applied to a "mobile bearing" design in which
the tibial bearing component is movable in vivo with respect to the
tibial baseplate. In mobile bearing designs, the periphery of the tibial
bearing component will generally be smaller than the periphery of the
tibial baseplate, similar to certain embodiments described above.

[0128] Femoral component 60 may be affixed to a distal end of femur F, as
appropriate, using any conventional methods and/or components. Exemplary
surgical procedures and instruments for such affixation are disclosed in
the Zimmer Surgical Techniques, incorporated by reference above. Femur F
and tibia T may then be articulated with respect to one another to ensure
that femur F, femoral component 60 and/or adjacent soft tissues do not
impinge upon tibial baseplate 12 and/or tibial bearing component 14 in
deep flexion, such as at a flexion angle β of 155° as shown
in FIG. 7. When the surgeon is satisfied with the location, orientation
and kinematic profile of tibial prosthesis 10, the knee replacement
surgery is completed in accordance with conventional procedures.

[0129] While this invention has been described as having an exemplary
design, the present invention can be further modified within the spirit
and scope of this disclosure. This application is therefore intended to
cover any variations, uses, or adaptations of the invention using its
general principles. Further, this application is intended to cover such
departures from the present disclosure as come within known or customary
practice in the art to which this invention pertains and which fall
within the limits of the appended claims.

Patent applications by Jeff C. Blaylock, Fort Wayne, IN US

Patent applications by Katherine M. Rettig, Cincinnati, OH US

Patent applications by Raymond C. Parisi, Wakarusa, IN US

Patent applications by ZIMMER, INC.

Patent applications in class Having member secured to femoral and tibial bones

Patent applications in all subclasses Having member secured to femoral and tibial bones